The present invention relates to a novel extruded continuous thermoplastic sheet material having a unique combination of physical properties and surface appearance highly suitable for use in electrical applications, such as in the production of printed circuit boards. The present invention particularly relates to an extruded polyalkylene terephthalate sheet material having a combination of physical properties and surface appearance suitable for use in the manufacture of circuit boards and in other electrical applications. The instant invention also relates to a process and apparatus for use in the manufacture of the aforementioned extruded sheet materials.
As used herein, the term "sheet material" is used in a generic sense, and includes both sheet and film.
Conventionally, printed circuit boards are manufactured from laminated sheets of thermoset phenolic polymers and epoxy-glass systems. The laminated phenolic sheets are die cut into an electrical component having a desired size and shape, called a substrate. An electrical circuit is then applied to the substrate. Due to the very intricate shapes which these substrates must possess, a large amount of scrap in produced. While phenol laminates and epoxy-glass systems possess the necessary combination of physical properties and surface appearance essential for use in electrical components such as circuit boards, phenol laminates and epoxy-glass systems are not reprocessable due to their thermoset nature. The use of these materials in the manufacture of circuit boards has thus been accompanied by an undesirably large volume of costly unusable scrap.
One approach to this problem has been to injection mold circuit board substrates from filled thermoplastic polymers. As is well known to those skilled in the art, a large number of the filled thermoplastic polymers, and particularly the glass-filled polyesters, and polyamides, possess the necessary combination of heat resistance, heat distortion temperature, flexural modulus, and flammability necessary for use in high temperature applications, such as electrical applications. The large expense and complexity associated with injection molding, however, has rendered injection molded substrates even less desirable for use in circuit boards than stamped phenol laminates.
It has also been suggested that circuit boards be manufactured from extruded sheets of filled thermoplastic polymers. Extrusion has the advantage of being a continuous and relatively inexpensive process for producing filled thermoplastic sheet materials. Heretofore, however, the prior art has been unable to develop an extruded sheet of filled thermoplastic polymer suitable for this purpose. A particularly glaring deficiency of prior art extruded sheet of filled and particularly glass-filled thermoplastic polymer has been that this material has possessed a very rough and uneven surface which has severely limited its commercial use, and particularly its use in circuit boards and other electrical components.
This inability of the prior art to develop a satisfactory extruded sheet of filled thermoplastic material has been particularly pronounced with respect to filled polyalkylene terephthalate compositions, and especially glass-filled polybutylene terephthalate compositions. As is well known to those skilled in the art, glass-filled polybutylene terephthalate compositions, such as those described in U.S. Pat. Nos. 3,764,576; 3,801,530; 3,873,491; 3,814,725; 3,903,042; 4,052,356; 4,123,415; and 4,124,561, possess a unique combination of physical properties which render these compositions ideally suited for use as circuit board substrates and in other electrical applications. While U.S. Pat. Nos. 3,742,087 and 3,580,964 suggest that filled polybutylene terephthalate compositions may be extruded into sheet materials, prior art attempts to produce such a material have resulted in a low melt strength, commercially unacceptable material having a very rough, roller-coaster surface. Prior art attempts to produce an extruded polybutylene terephthalate sheet material have been so unsuccessful in fact that an extruded glass-reinforced polybutylene terephthalate sheet material is presently unavailable on the market, despite a very great demand for such a material.
Another approach to the problem of producing a filled thermoplastic sheet material is disclosed in U.S. Pat. Nos. 2,950,502 and 3,074,114. In each of these patents, a filled thermoplastic sheet material is formed by calendering a plasticized admixture of filler material and thermoplastic polymer into a sheet and then polishing the resulting sheet with heated rolls. In U.S. Pat. No. 2,950,502, a layer of a silicone compound is employed to prevent sticking of the sheet to the polishing roll. Such processes are undesirable for use in the production of glass-filled sheet materials, however, since the use of calendering to form a sheet from the admixture of thermoplastic polymer and glass fiber, in contrast to extrusion, results in a substantial amount of undesirable glass fiber breakage. Moreover, the process of U.S. Pat. No. 3,074,114 is unsuitable for use with polymers that require drying, and employs such a high roll speed that a good surface appearance cannot be obtained. Each of these patents is basically concerned with producing wear surfaces for flooring having good color and pattern differentiation, and is not directed to producing filled thermoplastic sheets suitable for electrical uses.
U.S. Pat. No. 2,061,042 also discloses that a non-filled cellulose ester sheet material having a more uniform thickness may be formed by interposing a breaker plate between the screw and sheet die of an extrusion apparatus to create a pressure drop in the flow of cellulose ester and to develop a more uniform die pressure. The prior art has yet to develop, however, a suitable process for producing extruded, filled thermoplastic sheet materials.
Accordingly, there exists a great need in the art to develop an extruded thermoplastic sheet material suitable for use in electrical applications, as well as a process suitable for the production of such material.